Instagram is essentially a social network, with an emphasis on sharing photographs. (Think of Facebook, but instead of saying "just had a delicious burrito for lunch," you'd post a picture of it and caption it "my delicious lunch burrito.") Users have followers, and they can choose who to share their content with and who they want to communicate with.
Since we're primarily interested in how tweens use it here, I asked my 11-year-old about it. She doesn't have an account, but has friends who use it in various ways.
They share photos of funny things, and really cool things that happened to them, stuff like that. It's just a way to share funny and cool information with each other. You can tell people that stuff they're doing is awesome. You're able to give a lot more positive feedback than if you're just emailing back and forth. Most of the time you don't give negative feedback.
(Why would you not give negative feedback?) Well, you want to make feel better and happier. If they share something like "I'm so sad," you can help them be happier again. I guess there will be people who are rude and stuff, but, yeah.
That last part particularly piqued my interest. Research has shown that there are some positives to social media for tweens and teens (enhanced communication, strengthening social connections, helping develop a sense of self and place in the world), but there are also plenty of negatives (cyberbullying, “Facebook depression,” exposure to inappropriate content).
Knowing my daughter's circle of friends, I'm fairly confident that their interactions would be as friendly online as in person, and so I believe her when she says most of the interactions are positive. This is very dependent on the kids and friends, though; my friend's daughter got an Instagram account "because everyone has one!" After weeks of increasingly rude crap from her so-called friends (mostly "lol u weren't at this" pictures from parties she hadn't been invited to), the account got deleted. She's welcome to have it back when she's a bit older, with stronger emotional maturity and also some new friends.
There is a good article, "What parents need to know about Instagram," published by Media Smarts: I particularly liked, "T.H.I.N.K. before you comment on a friend’s photo: is it True, Helpful, Inspiring, Necessary, Kind?" My primary followup advice would be to disconnect from anybody who doesn't T.H.I.N.K. before posting.
To dig into the time sink question a bit, I have to turn back to the 11-year-old's anecdotes:
(Do you know anybody who uses it too much?) My friend M. spends way too much time on there. She's constantly on it, like all day, at [day camp], at home, she does it when she's eating, she does it in the bathroom, which, it's really weird. She just uses it way too much.
(How about any friends who use it sensibly?) L. is very consistent, she only goes on there at most thirty minutes a day. She only shares really cool pictures. One time she shared a picture of really cool gymnastics moves since she's really accomplished, but she doesn't share everything that happens every single day. Just important stuff.
So for a tip that is specific to Instagram, only share relatively important things. But, if he's viewing pictures shared by friends rather than sharing pictures he creates himself, there really aren't recommendations that are different from general Internet recommendations.